Aim
Cardiovascular diseases (CVD) are the leading cause of death worldwide emphasizing the importance of CVD research. The ventricles and atria of the heart display a distinct contractile behaviour and metabolic activity which are both affected in CVD. The cardiac ability to switch between different substrates, referred to as metabolic flexibility, is a hallmark of myocardial function and is partially impaired in heart failure. However, whether the atria and ventricles differ in their metabolic flexibility is largely unknown. Here, we aim to develop a human in vitro model to analyze differences in metabolic flexibility between atrial and ventricular cardiomyocytes using engineered heart tissue (EHT).
Methods
RNA sequencing data from human healthy left ventricular (LV) and left atrial (LA) heart tissue was retrieved from the GEO public data repository (GSE112339). EHTs were generated using ventricular and atrial hiPSC-derived cardiomyocytes. EHTs were analyzed for their spontaneous contractile development over a period of 4 weeks. Electrical stimulation was performed at 3 Hz to compare contractile force independent of beating frequency. To assess metabolic flexibility, EHTs were exposed to media restricted to glucose or fatty acids, followed by contractile function measurement. Metabolic behaviour was further characterized by performing metabolic tracer uptake, mitochondrial respiration analysis, and gene expression analysis.
Results
RNA sequencing data from healthy human LV and LA tissue revealed that KEGG pathways describing contractile and metabolic function are downregulated in LA. Contractile analysis showed higher beating frequency and lower contractile force in atrial versus ventricular EHTs. Lower contractile force was also observed in atrial EHTs during electrical stimulation at 3 Hz. When exposed to media restricted to fatty acids, atrial EHTs showed a reduction in contractile force which was not observed in ventricular EHTs. Media restricted to glucose had no effect on contractility in either atrial or ventricular EHTs. Exposure to radioactively-labelled metabolic tracer revealed less uptake of 3H-deoxyglucose and 14C-oleic acid into atrial EHTs compared to ventricular EHTs. Genes involved in glucose and lipid metabolism were lower expressed in atrial EHTs. While mitochondrial to nuclear DNA ratio suggested a similar amount of mitochondria in atrial EHTs versus ventricular EHTs, mitochondrial respiration analysis showed a lower respiratory activity of atrial EHTs with reduced respiration during activated oxidative phosphorylation.
Conclusion
Atrial and ventricular EHTs display a distinct contractile and metabolic behaviour in line with RNA sequencing data from healthy human heart tissue. Atrial EHTs are characterized by lower contractile force combined with a reduced metabolic activity. Reduced force in atrial EHTs after exposure to media restricted to fatty acids suggests a lower metabolic flexibility compared to ventricular EHTs.
